Traveling wave amplifier



van,

May 3, 1960 c. K. BIRDSALL 2,935,640

TRAVELING WAVE AMPLIFIER Filed March 24, 1954 2 Sheets-Sheet 1 May 3, 1960 c. K. BIRDsALL TRAVELING WAVE AMPLIFIER 2 Sheets-Sheet 2 Filed March 24, 1954 fw. 5@ w TRAVELING WAVE AMPLIFIER Charles K. Birdsall, Venice, Calif., assigner to Hughes Aircraft Company, Culver City, Calif., a corporation of Delaware Application March 24, 1954Serial No. 418,300 7 Claims. (Cl. S15-3.5)

This invention relates to microwave amplifiers and more particularly to a device for increasing the power output and the upper frequency limit of traveling-wave type amplifiers.

Traveling wave tube amplifiers are employed to achieve amplification by the interaction between an electron stream and a traveling wave. Conventional structure of these amplifiers normally includes a helical conductor disposed within an evacuated envelope along the path of the electron stream, the propagating characteristics of the helix being such that a propagated wave has electric field components parallel to the iiow of stream electrons and a phase velocity approximately equal to the stream velocity. The electrons of the stream thereby interact with the propagated wave to increase its amplitude as it progresses along the path.

In order to produce maximum amplication of the propagated wave, it is necessary to maintain the stream at a velocity slightly greater than that of the wave. The velocity of the stream is naturally determined by the magnitude of the direct-current voltage supplied between the electron stream source and the helix and the power output of the traveling wave tube is directly dependent on this voltage. Thus, in the case that it is desirable to increase this voltage to increase the power output, it also becomes necessary to increase the phase velocity of the propagated wave. Certain known methods have been employed at lower frequencies to solve .this problem such as, for example, increasing helix pitch or reducing helix diameter. Neither of these solutions presently appears to be practicable because of certain high frequency requirements which are imposed upon the dimensions of a helix, e.g. when helix pitch is increased, stream electrons are exposed between helix turns to helical areas of glass. Incumbent electron leakage from the stream to the tube envelope produces undesirable velocity modulation of the stream and periodic stream defocusing which prevent the production of any appreciable gain. Helix diameter is preferably not reduced to increase wave phase velocity as this would decrease the cross sectional area of the helix which would necessitate a decrease in the stream current and thus reduce the power output capabilities of the tube.

In addition to the above, it is known that an increase in the stream velocity through a particular helix increases the frequency at which backward wave oscillations occur. Self-oscillation of this character disturbs normal operation to such an extent that-a tube becomes substantially useless at this frequency. For this reason, the frequency at which backward wave oscillations occur is designated as a critical frequency. It is generally not feasible to employ a traveling wave tube to amplify frequencies above the critical frequency.

It is therefore evident that the power gain may be substantially increased and the critical frequency raised in a traveling wave tube by employing the present invention to increase the forwardwave phase velocity and helix voltage. This increase in wave phase velocity in the ice traveling wave tube is structurally produced by a plurality of conductive rings which are arranged in a spaced relationship along and disposed concentrically about the tube helix. The introduction of such conductors decreases the series inductance of the wave transmission path which, in turn, increases the phase velocities of forward waves. A pnlrality of these conductors disposed along the complete active length of the helix also has the eiect of increasing the normal dispersion of the helix, that is, the rate of the decrease in phase velocity for increasing frequencies.

It is also desirable to increase wave phase velocity throughout the portions of traveling wave tubes wherein reliected waves are attenuated in order to prevent selfoscillation. This attenuation is accomplished by a lossy material which is generally coated onto the tube envelope about the helix. In addition to attenuating however, the lossy material also decreases the phase velocity of the wave propagated by the helix which causes an appreciable reduction in both gain and bandwidth. The gain and bandwidth disadvantages imposed by the use of an attenuating material may be substantially eliminated by compensating for the nonuniformity in wave phase velocity existing throughout the helix by disposing the conductive rings of the present invention about the transmission path exclusively along the attenuator region to increase the phase velocity of the wave to its normal velocity as it is propagated through that region.

It is therefore an object of the invention to provide means for increasing the phase velocity of a forward wave in a traveling wave tube.

it is another object of the present invention to provide apparatus for increasing the power output and raising the maximum operating frequency of a traveling wave tube amplifier.

It is a further object of the invention to provide means for increasing the normal dispersion of a helical slow wave structure.

It is a further object of the invention to provide means for compensating for the nonuniforrnity in the phase velocities of waves propagated along a conductive helix through an attenuator region by increasing the phase velocities of the waves within that region.

A The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from tbe following description considered in connection with the accompanying drawings in which several embodiments of the invention .are illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only, and are not intended as a definition of the limits of the invention.

Fig. 1 is a sectional w'ew of a traveling wave tube ernploying one embodiment of the present invention;

Fig. 2 is a partial broken section of an alternative embodiment of the invention;

Fig. 3 is a phase velocity characteristic plotted as a function of frequency which is explanatory of the operation of the present invention;

Fig. 4 is a sectional view of a traveling wave tube employing the device of the present invention in conjunction with an attenuator region along a portion of the traveling wave transmission path; and

Fig. 5 is a partial broken section of an alternative emp bodiment of the invention shown in Fig. 4.

Referring to Fig. l, there is shown a traveling wave tube ampliier for amplifying microwave signals comprising an elongated evacuated envelope having an enlarged portion at the left extremity thereof, as viewed in the drawing, for housing an electron gun 101. Elec- 3 tron gun 101 produces a stream flow of electrons which is directed along a predetermined path that lies on the longitudinal axis of elongated envelope 100.

A solenoid 121 is axially positioned symmetrically about the complete length of envelope 100 and energized by appropriate direct current by means of a connection across a potential source, such as a battery 122, to produce a magnetic field which may be of the order of 600 to 1000 gauss running axially along the entire length of the tube. The purpose of this magnetic field is to keep the electron stream focused or constrained while traversing the path along the longitudinal axis of envelope 100.

Electron gun 101 comprises a cathode 102 with a heating element 103, a focusing electrode 104, and an accelerating electrode S. Heater' 103 is connected across a source of potential, such as battery 106, the negative terminal of which may be connected to a cathode 102. Focusing electrode 104 is of the type known as a Pierce electrode which has a frusto-conical shape with a surface of revolution about 671/2 mechanical degrees from its axis of symmetry. The space charge effect of an innitely large stream is thereby simulated and the electrous emitted from cathode 102 are focused to a solid cylindrical electron stream. The particular angle with the axis of symmetry enables this effect to be produced by focusing electrode 104 by maintaining it at zero volts relative to the potential of cathode 102 which may be effected by a connection thereto from electrode 104. Cathode 102 is then connected to the negative terminal of a source 107, the positive terminal of which is connected to ground. Electrode 105 is maintained at a potential positive with respect to cathode 102 by a connection to ground.

Disposed concentrically about the electron stream path in the direction of ow are a matching ferrule 103 connected over a lead 109 to a helix 110, which is, in turn, connected over a lead 111 to a matching ferrule 112. Helix 110 and ferrules 108 and 112 are maintained at the same potential as electrode 105 by means of a suitable connection thereto. A collector electrode 113 for intercepting and collecting the stream electrons is disposed at the right extremity of envelope 100, as viewed in the drawing and is maintained at a potential of the order of 200 volts positive with respect to the potential of ferrule 112 in order to prevent the return of secondary electrons thereto by a connection to the positive terminal of a battery 114, the negative terminal ofV which is connected to ground.V 4

An input to the tube is provided by an input waveguide 115 which symmetrically encloses a portion of envelope 100 that is coextensive with the lead 109 andhas a shorted termination 116 located one-quarter of a guide wavelength therefrom in order to effect optimum coupling from the waveguide 115 to the helix 110. A sleeve 117 is disposed concentrically about envelope 100 coextensively with ferrule 10S and extends from and is electrically connected to the side of waveguide 11S nearest electron gun 101. Both ferrule 108 and sleeve 117 are of an appropriate length to produce a virtual shorting plane at the inner surface of waveguide 115 so that substantially all of the energy is directed along helix 110 for modulating the electron stream.

An output waveguide 119 having a sleeve 120 is disposed about the envelope 100 coextensive with the lead 111 and ferrule 1.12, respectively, in the same manner as was done for the input waveguide 115.

During the operation of the tube, helix 110 propagates electromagnetic energy at a uniform velocity along the longitudinal axis of envelope 100 in the form of a traveling wave. ln order to maintain this uniformity, it is necessary that helix 110 hold its shape, especially with respect to its pitch and diameter. This requirement restricts the composition` of the helix to certain suitable metals such as, for example, tungsten or molybdenum.

In accordance with the present invention, a plurality of rings 118, composed of a', highly conductive material such as, for example, copper or silver, are disposed concentrically about the narrow portion of envelope coextensive with helix 110. These rings normally have a maximum cross sectional dimension which is much smaller than their radiusV of revolution. The number of rings per unit tube length is preferably about equal to the number of turns per unit length of helix and may, therefore, be spaced accordingly. The diameter of rings 118 need not be any certain absolute value; however, they have a more pronounced effect with respect to increasing the phase velocity characteristics and dispersion of helix 110 as the ratio of ring diameter to helix diameter approaches one. Rings 118, in fact, have no substantial effect upon these factors when the ratio is greater than five.

An alternative embodiment of the present invention is shown in Fig. 2 wherein onlyV a portion of the amplifying section of the tube is shown. This embodiment comprises a second conductive helix 140 disposed on the outside of evacuated envelope 100 coextensive with the wave propagating helix 110. This helix 140 preferably has of the order of l0 turns per each turn of helix 110 and a diameter as near that of helix 110 as is consistent with a practical construction of the tube. Any nonmagnetic conductive metal is suitable for the helix 140.

As is apparent from the foregoing embodiments, the present invention may be practiced generally by uniformly disposing a plurality of conductors transverse to the longitudinal axis of the helix 110 and adjacent to the turns thereof. There are obviously numerous equivalent structures that accomplish this result and it is assumed that these structures fall within the scope of the teachings of this specification.

In the operation of the tube of the present invention, a microwave signal to be amplified is impressed on input waveguide whence it is coupled to helix 110 which propagates the signal as an electromagnetic wave along the longitudinal axis of envelope 100 at a velocity substantially less than the velocity of light but at a velocity substantially greater than normally employed because of the effect of conductors 118. The velocity of the electron stream is accordingly increased so as4 to effect constructive interaction with the electromagnetic wave thereby amplifying the signal. This amplied signal is coupled to the output waveguide 119 where it is available as the amplified output signal from the tube. The result of the foregoing operation is, however, an abnormally high power output which is obtained due to the increase in the electron stream velocity which is made possible by the increase in the normal magnitude of the wave phase velocity effected by rings 118 of the present iuvention.

The eifect produced by rings 118 may be better understood by making an analogy between helix 110 and a conventional transmission line having series inductance and shunt capacitance distributed uniformly along its length. As is commonly known, the phase velocity of a waverpropagated by the transmission line varies inversely with the square root of the product of this inductance and capacitance. Thus it is evident that any decrease in the series inductance or shunt capacitance has the effect of increasing the phase velocity of the propagated wave.

Noting that inductance is a measure of flux linkages per ampere of current, the inductance is decreased by the concentric conductors of the present invention in that a current is induced in them which flows and generates magnetic ilux that tends to cancel the original flux linkages. It is evident that the closer the concentricvconductors are to the turns of the helix, the larger the currentthat will `be induced in, them andthe more theV ux linkages thatara 6211.196116@v T0 illustrate the effect 0f this spacing more clearly, reference is made to Fig. 3 wherein the phase velocity versus frequency characteristic for several ring to helix diameter ratios are shown. In this figure, lines 150, 152, 154 and 156 represent the phase velocity versus frequency characteristics for the diametral ratios 1.05, 1.10, 1.20, and 2.00, respectively. Inspection of these representative characteristics reveals that diametral ratios of 1.05 and 1.10 both produce a substantial increase in the phase velocity and dispersion of the propagated Wave throughout the operating-range of the tube. Also in Fig. 3 is a line 158 which represents the phase velocity versus frequency characteristic for a backward wave on the helix 110. As shown, the intersection of the backward wave characteristic 158 with the forward wave characteristics 150, 152, 154 and 156 extends to higher and higher frequencies as the -diametral ratios are decreased, thus raising the usable frequency range of the tube.

The curves of Fig. 3 also ably illustrate how power gain may be increased, e.g. at frequency fo by employing a 1.05 diametral ratio, phase velocity is increased 100% and helix voltage may therefore be increased proportionately to increase power gain.

It has been found that an increase in the dielectric constant between the helix 110 and the concentric rings 118 produces an increase in the effective shunt capacitance of the helix 110 which would decrease the phase velocity of a propagated wave. In order to minimize this effect, it is preferable to use a material for envelope 100 that has a low dielectric constant.

An alternate embodiment of the present invention is shown in Fig. 4 wherein the concentric rings 118 are replaced with a resistive coating 160, which is disposed about a portion of envelope 100 along the helix 110, and a number of annular conductors 162 which are spaced coextensive with and -disposed concentrically about the coated portion of envelope 100. Resistive coating 160, which may be of carbon black, is applied on the outside of envelope 100 about the center turns of helix 110. Coating 160 is employed to attenuate waves which may be reflected from the output end of the tube because of an impedance mismatch which normally occurs there. This mismatch cannot be avoided in practice because a traveling wave amplifier has such an extremely large bandwidth that a perfect termination for all frequencies is impossible. Resistive coating 160 thus decreases the tendency of the tube to break into a type of selfoscillation which usually accompanies this reflection of microwaves. Forward waves traveling along with the stream electrons are not appreciably attenuated since they are propagated primarily by the stream. It is to be noted that there are numerous methods of attenuating an electromagnetic wave and that the method described is merely for the purpose of illustration.

Even though coating 160 serves an extremely useful purpose, there are, however, certain inherent disadvantages that are attendant upon its employment. In the first place, the use of coating 160 decreases the phase velocity of waves as they are propagated through the region coextensive with the coating. This effect causes gain to vary appreciably with frequency and thereby restricts tube operation to unfavorably narrow bandwidths. The use of coating 160 is also known to appreciably decrease the net maximum gain of a tube. These operating characteristics may be minimized by employing the present invention to cancel the decrease in phase velocity of the wave within the helix region of attenuation, thereby making the phase velocity of the wave uniform throughout the wave transmission path. The disadvantages of using resistive coating 160 are, in fact, substantially eliminated by the employment of conductors 162 which function in the same manner as concentric rings 118 of Fig. l.

Alternatively, the attenuation and phase velocity increasing functions may be combined as shown in Fig. 5. Referring to this figure, a series of lossy rings 164 are disposed concentrically about a portion of the amplifying section in lieu of the resistive coating 160 and concentric rings 162 of Fig. 4 to attenuate the electromagnetic portion of the traveling wave without changing its phase velocity.

What is claimed is:

1. A traveling-wave tube comprising a conductive helix for propagating an electromagnetic signal wave along a predetermined path, electron beam means for projecting a beam of electrons within and in interacting relation with said conductive helix, and a plurality of highly conductive nonmagnetic metallic loops direct current insulated from said helix and disposed about and spaced uniformly along at least a portion of said predetermined path and spaced apart a distance not substantially greater than the `distance between adjacent turns of said conductive helix for increasing the phase velocity of said signal wave, the ratio of the outer diameters of said conductive loops and of said helix being less than approximately 1.2.

2. A high power traveling-wave tube comprising: a conductive helix for propagating an electromagnetic signal wave along a predetermined path; means for increasing the phase velocity of said wave comprising a series of highly conductive, nonmagnetic, axially separated, annular rings of substantially equal diameter and less than approximately 1.2 times that of said helix positioned concentrically about and spaced uniformly along the longitudinal axis of said helix by a spacing approximately equal to the pitch of said helix; means for producing an electron stream, and means for directing said electron stream within said helix at a velocity substantially equal to the increased phase velocity to effect amplification of said signal wave.

3. A high powered traveling-wave tube comprising: a conductive helix for propagating an electromagnetic signal wave along a predetermined path; means for increasing the phase velocity of said wave along said path including an additional highly conductive nonmagnetic helix positioned concentrically about the longitudinal axis of and having a substantially greater number of turns per unit length than said helix for propagating the electromagnetic signal wave and being direct current insulated therefrom.

4. A high powered traveling-wave tube comprising: a conductive helix for propagating an electromagnetic signal wave along a predetermined path; means for increasing the phase velocity of said wave along said path including an additional highly conductive nonmagnetic helix positioned concentrically about the longitudinal axis of and having of the order of ten turns per each turn of said helix for propagating the electromagnetic signal Wave and being direct current insulated therefrom.

5. A traveling wave tube comprising a conductive helix for propagating an electromagnetic signal wave at a uniform velocity along a predetermined path, an output transmission line coupled to one extremity of said conductive helix, means for producing an electron stream, means for directing said electron stream within said helix towards said one extremity at a velocity to amplify said signal wave, whereby a part of the energy of said signal wave is reected at said output transmission line, a resistive material disposed along a portion of said predetermined path to attenuate the reflected part of said signal wave, whence the phase velocity of said signal wave is decreased throughout said portion of said path, and a series of highly conductive, nonmagnetic metallic loops disposed at uniform intervals along the longitudinal axis of said helix substantially concentric therewith and adjacent to the turns thereof along said portion of said predetermined path to increase the phase velocity of said Wave to said uniform velocity, said conductive loops being direct current isolated from said helix and said Y 7 transmission line, and-havingxa icross-sectional dimension much smaller than their'loop radius, andthe ratio of the outerdiameters of said loopsrtothat of said helix being lessthan approximately 1.2.

6. The traveling-wave tube as dened in claim 5, wherein said Yseries ofrconductive loops are annular in shape and have equal diameters.

7. A traveling-wave tube comprising: a conductive slow-wave structure for propagating an electromagnetic signal wave along a predetermined path, external transmission line means coupled to said slow-wave structure, electron beam means for projectingv a beam of electrons within and in interacting relation with said` conductive slow-wave structure, means to reduce the series inductance to thereby increasethe phasevelocity of saidl signal waves alongfsaid slow-wave structure, the latter means including a plurality of 'spaced highly conductive nonmagnetic loopsy disposed'substantially coaxially with said slow-wave structure f externaly thereto, said conductive loops havingan outer dimension, transverse to said path,

3 less than approximately 1-.2 times that of said slow-wave structure and: being direct current insulated from said slow-wave structure and said transmission line means.

References Citedin the file-of this patent UNITED STATES PATENTS 2,300,052' Lindenblad Oct. 27, 1942 2,588,832 Hansell Mar. 11, 1952 2,602,148 Pierce July 1, 1952 2,626,371 Barnett et al. Jan. 20, 1953 2,707,759 Pierce May 3, 1955 2,753,481 Ettenberg July 3, 1956 2,773,213 Dodds Dec. 4, 1956 21,809,321 Johnson et al.' Oct. 8, 1957 2,811,673 Kompfner Oct. 29, 1957 2,813,221 Peter Nov. 12, 1957 2,867,745 Pierce Jan. 6, 1959 FOREIGN PATENTS 992,048 France June 27, 1951 

